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Well I am no expert here but you do not need a sight glass on an AC circuit for it to function. Indeed as a maintainer you can diagnose and charge the system without it - but it does help. I suppose cost driven production may hhave seen it excluded on some cars.
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It's also a Ford thing.
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Now be very careful guys.
Sight glassses were used only for pre94 cars running with R12, and CAN'T be used reliably for post 93 running R134a refrigerant. The simple reason is that with R134a there will nearly always be bubbles, so if you fill it until the glass goes clear, then the system will be overfilled and on a warm day the system will go pop. If you do have an old car (pre93), then in order to use the glass it needs to be at least 18*C outside. But as R12 is no longer available, you can't top it up anyway, and if you use an alternative (or R134a), then because it's not R12, you still can't use the sightglass.
They're still fitted to some cars for a couple of reasons. Some countries are still allowed to use R12, and some designs haven't changed. The only way to tell rerfrigerant qty is to recover it. There is no other way, and the dealer should know this.
There is one exception though, and that is the very latest Toyotas, where, due to system design, sightglasses have made a comeback.
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